The trend toward ever greater device densities, smaller minimum feature sizes, and smaller separations in integrated circuits imposes increasingly stringent requirements on the basic fabrication steps of masking, film formation (by deposition or growth), doping and etching. Ultra large scale integrating (ULSI) devices require the fabrication of circuits on the nanometer (nm) scale. For certain applications in such circuits, silicon nitride films offer advantages over silicon oxide films. For example, as a collar material for storage trenches, a film of silicon nitride 30 nm thick can provide comparable dielectric properties to a 100 nm silicon dioxide film. In addition, because stable fluorinated silicon nitride is not permeable to oxygen, the surface of the substrate (usually silicon) is protected from oxidation. Silicon nitride films that are stable and conformal would be particularly useful for low temperature side wall spacers for sub-micron gates, for high temperature annealing caps for GaAs dopant activation and for interlevel dielectric materials.
However, the advantages of a silicon nitride film can only be realized if a thin film can be made by a process that is compatible with the technology for manufacturing ULSI devices. The process must avoid high temperatures and incompatible chemistry. Chemical vapor deposition processes for producing silicon nitride films are known.
U.S. Pat. No. 4,668,365 (Foster et al.) discloses a plasma CVD reactor and an associated process using a magnetic field to provide silicon nitride films. This process uses silane as a source of silicon and molecular nitrogen as the source of nitrogen.
U.S. Pat. No. 3,485,666 (Sterling et al.) discloses a method of directly depositing an electrically insulating, amorphous, coherent, solid layer of silicon nitride produced from silane and ammonia.
U.S. Pat. No. 4,786,612 (Yau et al.) discloses a plasma enhanced CVD process for fabricating a silicon nitride resistor in a semiconductor device. The process uses a mixture of silane, nitrogen and ammonia.
U.S. Pat. No. 4,504,518 (Ovshinsky et al.) discloses a plasma enhanced CVD process for producing a silicon nitride film which uses microwave radiation to generate the plasma. Mixtures of silane and nitrogen or ammonia are disclosed as reaction gases and mixtures of silicon tetrafluoride and nitrogen or ammonia are also disclosed.
Shibagaki et al. [Japanese Journal of Applied Physics, 17, suppl 17, 215-221 (1978)] discloses the deposition of the silicon nitride film by the reaction of silane with molecular nitrogen using microwave energy to generate the plasma.
Silicon nitride films of the foregoing art have been limited in their utility by the presence of silicon hydrogen bonds in the silicon nitride film. Some of the silicon hydrogen bonds are relatively weak and tend to dissociate with subsequent migration of hydrogen through the silicon nitride material. In many of the films of the art the concentration of hydrogen in the deposited silicon nitride can be as high as 25-35 atom percent. The presence of hydrogen in films that are deposited relatively early in the integrated circuit fabrication process, and the hydrogen diffusion which results during subsequent fabrication, can cause non-uniform electrical characteristics and adhesion problems at interfaces of the film.
There has thus been a strong incentive to produce silicon nitride having a low hydrogen content or having strongly bonded hydrogen that is not mobile. However, the removal of hydrogen by annealing appears to have led to chemical instability such that the film exhibits film stress and poor adhesion. The drawback of this approach to lowered hydrogen has led to a search for plasma processes which employ gases that reduce or stabilize hydrogen in the silicon nitride. The incorporation of fluorine into the silicon nitride film obviates a number of these problems. Although a precise explanation for the combination of fluorine, hydrogen, nitrogen and silicon into silicon nitride layers is not available, it is believed that fluorine present in the plasma preferentially bonds with silicon in the silicon nitride material either preventing formation of silicon-hydrogen bonds or replacing such bonds with silicon-fluorine bonds. It would seem that providing an abundance of fluorine atoms would solve the problem. However the presence of excess fluorine in the plasma results in the creation of nitrogen-fluorine bonds and SiF.sub.2 linkages. The presence of nitrogen-fluorine bonds is not desirable because they lead to a substantial chemical degradation in the resulting silicon nitride film. Similarly SiF.sub.2 linkages are extremely sensitive to degradation by moisture. Clearly an appropriate ratio of silicon and nitrogen to fluorine is required to produce the desired stoichiometry.
U.S. Pat. No. 4,720,395 (Foster) discloses a thermal CVD process for forming a silicon nitride film. Disilane and NF.sub.3 in a mole ratio of 0.5-3.0 are employed as the sources of silicon, nitrogen, and fluorine. The films are characterized only by their refractive indices.
U.S. Pat. No. 4,704,300 (Yamazaki) discloses a plasma CVD process for forming a fluorinated silicon nitride film. Silicon fluoride is fed into the reactor in the form of SiF.sub.4 and converted in situ to a mixture of SiF.sub.4 and SiF.sub.2 which then reacts with nitrogen, ammonia or nitrogen fluoride to provide the silicon nitride film. The process does not employ silane as a reactant. The films are stated to have a surface density of 3.times.10.sup.11 charges/cm.sup.2 or less.
European Application 277,766 (Chang et al.) discloses a plasma CVD process for forming a silicon nitride film. The examples employ mixtures of silane, NF.sub.3, and nitrogen. The generic description indicates that hydrazine or ammonia may be used in place of NF.sub.3 and SiH.sub.4 may be replaced by Si.sub.2 H.sub.4 or chlorinated or partially fluorinated silanes. The films are stated to have breakdown voltages of 4 MV/cm and dielectric constants of 4 to 5.
The foregoing references disclose methods for producing fluorinated silicon nitride films, but all have drawbacks. The use of NH.sub.3, as suggested by Chang and by Yamazaki, leads to films having lower long-term chemical stability than films made from N.sub.2. The use of NF.sub.3, as suggested by Chang and by Foster, leads to etching of the substrate during the initial deposition step, which is usually undesirable. Moreover, the production of silicon-fluorine bonds using NF.sub.3 as the source of fluorine requires a multi-step dissociation and recombination reaction which is inherently less attractive than starting with silicon-fluorine bonds. The use of hydrogen fluoride as suggested by Chang is difficult to control; both NF.sub.3 and HF are highly corrosive materials.
There thus exists a need for an easily controlled, predictable CVD process for producing fluorinated silicon nitride films that are stable, conformal and compatible with other steps in the fabrication of integrated circuits.
The term silicon nitride as used herein means a composition of approximate stoichiometry Si.sub.3 N.sub.4 and fluorinated silicon nitride describes compositions of empirical formulae Si.sub.3 N.sub.x F.sub.y H.sub.z where x is from 3 to 4, y is from 0.01 to 1.15 and z is from 0.7 to 5. The generic term silane includes SiH.sub.4, Si.sub.2 H.sub.6, Si.sub.3 H.sub.8 and other silanes of empirical formula Si.sub.n H.sub.(2n+2). The generic term perfluorosilane includes SiF.sub.4, Si.sub.2 F.sub.6, Si.sub.3 F.sub.8 and other perfluorosilanes of empirical formula Si.sub.n F.sub.(2n+2).